Indole recovery process



Dec. 8, 1959 J, R, ANDERSQN ETAL 2,916,496

INDOLE RECOVERY PROCESS Filed May 9, 1955 ATTORNEY United States Patent ice 2916496 Y Y vPatented Dec. 8, 1959 indoles. This fraction may then be treated in any desired y manner to recover the indoles. Prior art methods in- 2 916 496 clude precipitation of the sodium or potassium salts. Inf doles may also be recovered from the indoles and aro- INDOLE RECOVERY PROCESS 5 matic hydrocarbons mixture produced by the invention by Application May 9, 195s, sei-n1 No. 501,112

9 claims. (cl. 26o- 319) This invention relates to a continuous extraction process whereby a mixture of indoles and hydrocarbons 1s separated into one or more fractions containlng the bulk of the hydrocarbons and a fraction containing the bulk Vof the indoles. More particularly the invention relates to the application of this method to the industrial products in which indoles occur, such as the neutral light oil from -coal hydrogenation.

`'Indole is used in the production of perfumes and in the synthesis of tryptophane, an essential amino acid. More widespread industrial use would be likely except for its scarcity and high cost. It occurs naturally in organic sources such as oil of jasmine, clove oil and the products of putrefaction of amino acids. The major commercial source at present is the coke industry, where indole is recovered from coal tar oil. A new source of supply,

however, is now being found with the commercial developl is a nitrogen containing compound similar in structure to the nitrogen base, it is neutral to weakly acid in nature and lresembles the aromatic hydrocarbons in many of its properties, which accounts for its being found in the neutral light oil fraction rather than with the nitrogen bases as might be expected.

In the past indole has been recovered from the appropriate fraction of coal tar oil by taking advantage of the weakly acidic properties of its N-H group. This involves precipitating the potassium or sodium salt of indole, separating the salt from the oil mixture, and then hydrolyzring the salt to give indole and sodium or potassium hydroxide. A number of German patents, including Nos. 223,304, 238,138 and 454,696, disclose this method, which is unsatisfactory in that multiple treatment of the indole fraction is necessary to obtain indoles of suiciently high quality, and each step must be carried out batchwise.

The present invention provides an improved continuous process whereby a mixture of indoles and aromatic hydrocarbons can be prepared, which mixture contains only a small proportion of aromatic hydrocarbons and is substantially free from aliphatic hydrocarbons and partially hydrogenated polynuclear compounds. Applied to the industrial products in which indoles occur, such as neutral light oil from coal hydrogenation, coke oven oil, or coal tar oil, or in fact to any hydrocarbon mixture containing indoles, the present invention yields an aromatic hydrocarbon fraction having a relatively high concentration of 'cover it.

employing the azeotropic distillation process disclosed in U.S. patent application Serial No. 507,168, concurrently led by James V. Murray, Jr., John D. Fales and Charles H. Young. Murray et al. teach that their azeotropic distillation is best applied to a mixture of indoles and aromatic hydrocarbons containing substantially no nonaromatic hydrocarbons or partially hydrogenated polynuclear compounds and this type of mixture is produced by the present invention. y

According'to the method of our invention, the neutral light oil or other mixture of indoles and hydrocarbons is rst extracted with a 1,2-alkylene carbonate containing from three to four carbon atoms. Ethylene carbonate is preferred butpropylene carbonate may also be used. The volume ratio of carbonate to indole-bearing oil may be from 1:20 to 20:1. Atmospheric pressure and a temperature up to about 10 degrees above the freezing point of ethylene carbonate, or roomtemperature in the case of propylene carbonate, are preferred, but temperatures Vup to the temperature where appreciable decomposition of the carbonate occurs are operable. The selectivity and capacity of the carbonate solvent may be adjusted, if desired, by the admixture with the carbonate of minor amounts of other substances, such as water. The extraction may be run as a single stage batch extraction but is preferably conductedas a multistage continuous extraction; vMechanical aids such as mixers-packing, baiiies and the like may be contained in the extraction apparatus. This carbonate extraction removes as extract substantially all the indoles present', together with the more highly aromatic hydrocarbons, such' as naphthalene. This extract represents a more concentrated indole mixture -than the neutral light oil or other mixtures from which t.was derived, and after removal of the'carbonate, the indole could be removed directly by azeotropic distillation as disclosed in the Murray et al. application, or by other suitablemeans. There is still, however, a large proportion of aromatic hydrocarbons present in admixture with the indoles and the removal of the carbonate at this stage is difficult, particularly in view of the tendency of the carbonate to decompose and be degraded during the distillation which is customarily employed to remove or reprocess. According to our invention the carbonate extract containing indoles and aromatic hydrocarbons is further extracted with a naphthasuch as n-heptane, petroleum ether or cyclohexane which removes the bulk of the aromatic'ghydrocarbons and leaves as residue the carbonate, some` residual aromatic hydrocarbons and most of the indoles, with less than live percent of the total indoles ordinarily being lost to the extract. The naphtha used for the extraction maybe any low-boiling nonaromatic hydrocarbons, preferably those boiling between 50 and 110 C. The solvent` to feed ratio for the extraction should be between' 1:20 and 20:1, with about 2:1 being preferred. The pressure and temperature should be such that the feed, the solvent, the extract and the ratiinate are all in the liquid state. Atmospheric pressure is ordinarily suitable, and room temperature is suitable if propylene lcarbonate is the alkylene carbonate used. If ethylene carbonate, which has a melting point of 'about 36 C. 'is -used,`the operating temperature'must be above the melting point. Temperatures of from 36 to 50 C. are preferred, although temperatures up to about 140 C., the temperature at which appreciable decomposition of ethylene carbonate occurs, may be used. The residue is then extracted with a low-boiling alkyl ether such as isopropyl ether or n-butyl ether which removes as extract the indoles and the residual aromatic hydrocarbons,`as Well as a small amount of the carbonate, and leaves the carbonate as residue which can be recycled. The solvent to feed ratio should be about 1:1. vPressure and temperature limitations are the same as those given immediately above for ythe naphtha extraction. The alkyl ether is then distilled from the extract leaving the indoles, the residual aromatic hydrocarbons and the small quantity of carbonate which had been dissolved by `the isopropyl ether. A nal extraction with Water Vis then employed to remove residual carbonate land leave vas product a mixture in aromatic hydrocarbons of a relatively high proportion of indoles. Indole-bearing oils also frequently contain phenolic compounds and nitrogen bases. Compounds of these types are not separated from the `indoles by the process of the invention and must be removed by established techniques.

It is to be understood that in speaking of extracting indole-bearing oils with an `alkylene carbonate in the present invention, We intend applying the process to all types of indole-bearing oils including those which contain little or no nonaromatic hydrocarbons. Wtih these -latter type of oils, the term extraction includes adding the carbonate to the oil to dissolve the indoles and aromatic hydrocarbons making up the oil; The carbonate will then serve as a solvent to hold the indoles while the aromatic hydrocarbons are extracted by the naphtha backwash.

The drawing is a oW sheet of the process of the invention. The mixture of idoles and hydrocarbons, such as neutral light oil, is introduced into an extractor 11 through 'line 12. The carbonate solvent is introduced into the extractor 11 through line 13. The extract from extractor ,11, comprising the carbonate extraction solvent, the indoles and the aromatic hydrocarbons is led out through line 14 into a second extractor '15, while the residue of aliphatic and more hydrogenated aromatic hydrocarbons is removed from the extractor .11 through line 16. In the case of oils containing little or no nonaromatic hydrocarbons where the carbonate will be used primarily as a solvent rather than as Van extracting agent and a solvent, the extractor 11 may .be eliminatedand the .carbonate solvent in line 13 and the mixture of `indoles and hydrocarbons `in line 12 may be .mixed directly and conducted through line 14 into the .second extractor 15. Naphtha is introduced into the second extractor 15 through line .17. After the naphtha extraction, the extract, comprising naphtha and the bulk of the aromatic hydrocarbons is removed through line 18 while the residue, comprising i carbonate, indoles and residual aromatic hydrocarbons is removed through line 19 and led into a third extractor 20.

A third solvent, isopropyl ether, is led into the third extractor 20 through line 21. After the ether extraction the extract, comprising isopropyl ether, indoles, residual aromatic hydrocarbons and remaining residual carbonate Vis led out of the third extractor 20 through line 22 and into a stripping still 23. The `residue of carbonate solvent is removed from the third extractor 20 through line 24 and is recycled through line 24 back to the carbonate feed line 13 of the irst extractor 11, for reuse in the process. 'Ihe distillate from the stripping still 23, comprising isopropyl ether, is removed through line 25 and .recycled through this line to 4the isopropyl ether feed line 21 of the third extractor 20, for reuse in the process.

The residue from the stripping still 23, comprising iudoles, residual aromatic hydrocarbons and remaining residual carbonate, is .removed through line 26 and led into a fourth extractor 27. Water as a solvent is introduced into the fourth extractor 27 through line 2.3 to wash out .residual carbonate. At this point, if necessary, any phenols or nitrogen bases present must be removed by conventional techniques such as Washing with aqueous caustic to remove the phenols and with an aqueous mineral acid to remove the nitrogen bases. As these techniques Will also remove or destroy any residual carbonate present, water washing to remove carbonate must be applied first if recovery of the residual carbonate is desired. The extract from the fourth extractor 27, comprising Water and residual carbonate, is removed through line 29 and may then be conducted to a carbonate recovery apparatus for recovery of the carbonate for eventual recycle to the process. The residue from the fourth extractor 27 is removed through line 30 and comprises the final product of the process, a concentrated mixture of indoles in aromatic hydrocarbons.

To complete the continuous process, a second stripping still 31 is provided for separation of the naphtha and aromatic hydrocarbon mixture from the second extractor 15. The extract from the second extractor 15, which comprises naphtha and the bulk of the aromatic hydrocarbons from the original feed to the process, is removed from the extractor 15 through line 18 and is conducted through line 18 to the second stripping still 31. The naphtha distillate from the stripping still 31 ,is Vremoved through line 32 and recycled through this line to the naphtha feed line 17 lfor reuse in the second extractor 15 of the process. The residue from the second stripping still 31, comprising a mixture of aromatic hydrocarbons, is removed through line 33 and can then be conducted to a suitable apparatus for recovery of the constituents of the mixture.

Example I Coal hydrogenation neutral light oil having a nominal boiling point range of to 260 C. and containing about one percent by Weight of indoles and about one percent by Weight of phenolic compounds was extracted iirst with ethylene vcarbonate, solvent at a solvent to feed ratio of two to one in a l2-stage, 2 inch inside diameter York Scheibel extraction column. The ethylene carbonate extract containing indoles, phenols and aromatic hydrocarbons Was back-extracted With heptane at a heptane to feed ratio to one to one in order to remove the bulk of the aromatic hydrocarbons in the extract.

The indoles, phenols and residual aromatic hydrocarbons were removed from admixture with ethylene carbonate in the residue from the heptane extraction by extracting them with isopropyl ether at an ether to feed ratio of one to one. This extract was then distilled to remove the isopropyl ether and Was extracted rst with Water to remove the residual ethylene carbonate which dissolved in the ether and then With sodium hydroxide to remove the phenols. The nal product after the sodium hydroxide extraction was a mixture of aromatic hydrocarbons containing about 12 percent by weight of indoles.

Example II Coal hydrogenation neutral light oil having a nom-inal boiling point range of 100 to 260 C. and containing about one percent by Weight of indoles and about one percent 'by Weight of phenolic compounds Was extracted iirst with ethylene carbonate solvent at a solvent to feed ratio which varied between one and 2.5 to one, using a 12-stage, 2-inch inside diameter York Scheibel extraction column. The .ethylene carbonate extract containing indoles, phenols and aromatic hydrocarbons was back-extracted with n-heptane at aheptane to feed ratio of two to one.

The indoles, phenols and residual aromatic hydrocarbons were removed from admixture with ethylene icarbonate `in `the residue from the n-heptane extraction by yextracting them with isopropyl ,ether yat an ether to feed v y l s 4the'ri distilled to remove the isopropylfethery andfwas extracted with sodium hydroxide to remove vthev p henolsa'nd residual `car- .bonate. The final product thus produced'was a mixture "'of aromatic hydrcarbons.containlng=about 32 percent by fweightfof'indoles. u

We claim: 1' A continuous Process for Producing a concentrated solution'of indoles in aromatic hydrocarbons from an indole-bearing oil containing aromatic hydrocarbons which comprises extracting said indole-bearing oil with a 1,2- alkylene carbonate containing from three to four carbon atoms to produce an alkylene carbonate-indoles-aro matic. hydrocarbons rst extract; back-extracting said rstextract with a naphtha to remove as extract the bulk of the aromatic hydrocarbons and leave as a second residue alkylene carbonate, indoles and aromatic hydrocar- -bons; extracting said second residue with a lower alkyl ether to produce an ether-indoles-aromatic hydrocarbons third extract; distilling said ether from said third extract, and washing said third extract with water to remove residual carbonate to leave as product said concentrated solution of indoles in aromatic hydrocarbons.

2. A continuous process for producing a concentrated .solution 'of indoles in aromatic hydrocarbons from an indole-bearing oil containing aromatic hydrocarbons which comprises extracting said indole-bearing oil with i ethylene carbonate to produce an ethylene carbonateindoles-aromatic hydrocarbons rst extract; back-extracting said rst extract with a naphtha to remove as extract which comprises extracting said indolebearing oil with propylene carbonate to produce a propylene carbonateindoles-aromatic hydrocarbons irst extract; back-extracting said first extract with a naphtha to remove as extract thebulk of the aromatic hydrocarbons and leave as a second residue propylene carbonate, indoles and aromatic hydrocarbons; extracting said second residue with a lower alkyl ether to produce an ether-indoles-aromatic I hydrocarbons third extract; distilling said ether from said third extract, and washing said third extract with water to remove residual carbonate to leave as product said concentrated solution of indoles in aromatic hydrocarbons.

4. A continuous process for producing a concentrated solution of indoles in aromatic hydrocarbons from an indole-bearing oil containing aromatic hydrocarbons which ycomprises extracting said indole-bearing oil with a 1,2-

alkylene carbonate containing from three to four carbon atoms to produce an alkylene carbonate-indoles-aromatic hydrocarbons lirst extract; back-extracting said rst extract with a naphtha to remove as extract the bulk of the'aromatic hydrocarbons and leave as a second residue alkylene carbonate, indoles and aromatic hydrocarbons; extracting said second residue with isopropyl ether to produce an ether-indoles-aromatic hydrocarbons third extract; distilling said ether from said third extract, and washing said third extract with water to remove residual carbonate to leave as product said concentrated solution of indoles in aromatic hydrocarbons.

5. A continuous process for producing a concentrated solution of indoles in aromatic hydrocarbons from an indole-bearing oil containing aromatic hydrocarbons aie-16,496

which comprises extracting said indole-bearing oil with ya 1,2-alkylene carbonate containing from three to four carbon atoms to produce an alkylene carbonate-indoles-aro- Imatic hydrocarbons lirst extract; back-extracting said first f extract with a'naphtha to remove as extract the bulk of 'the aromatic hydrocarbons and leave as a second residue alkylene carbonate, indoles and aromatic hydrocarbons; extracting said second residue with n-butyl ether to produce an ether-indoles-aromatic hydrocarbons third extract; distilling said ether from said third extract, and washing said third extract with water to remove residual carbonate to leave as product said concentrated solution of indoles in aromatic hydrocarbons.

6. A continuous process for producing a concentrated solution of indoles in aromatic hydrocarbons from an indole-bearing oil containing aromatic hydrocarbons which comprises extracting said indo1e-bearing oil with a 1,2- alkylene carbonate containing from three to four carbon atoms to produce an alkylene carbonate-indoles-aromatic hydrocarbons rst extract; back-extracting said first extract with n-heptane to remove as extract the bulk of the aromatic hydrocarbons and leave as a second residue alkylene carbonate, indoles and aromatic hydrocarbons; extracting said second residue with a lower alkyl ether to produce an ether-indoles-aromatic hydrocarbons third extract; distilling said ether from said third extract and washing said third extract with water to remove residual carbonate to leave as product said concentrated solution of indoles in aromatic hydrocarbons.

7. A continuous process for producing a concentrated solution of indoles in aromatic hydrocarbons from an indole-bearing oil containing aromatic hydrocarbons which comprises extracting said indole-bearing oil with a 1,2-alkylene carbonate containing from three to four carbon atoms to produce an alkylene carbonate-indolesaromatic hydrocarbons rst extract; back-extracting said first extract with petroleum ether -to remove as extract the bulk of the aromatic hydrocarbons and leave as a second residue alkylene carbonate, indoles and aromatic hydrocarbons; extracting said second residue with a lower alkyl ether to produce an ether-indoles-aromatic hydrocarbons third extract; distilling said ether from said third extract, and washing said third extract with water to remove residual carbonate to leave as product said concentrated solution of indoles in aromatic hydrocarbons.

8. A continuous process for producing a concentrated solution of indoles in aromatic hydrocarbons from an indole-bearing oil containing aromatic hydrocarbons which comprises extracting said indole-bearing oil with a 1,2- alkylene carbonate containing from three to four carbon atoms to produce an alkylene carbonateindoles-aromatic hydrocarbons irst extract; back-extracting said iirst extract with cyclohexane to remove as extract the bulk of the aromatic hydrocarbons and leave as a second residue alkylene carbonate, indoles and aromatic hydrocarbons; extracting said second residue with a lower alkyl ether to produce an ether-indoles-aromatic hydrocarbons third extract; distilling said ether from said third extract, and washing said third extract with water to remove residual carbonate to leave as product said concentrated solution of indoles in aromatic hydrocarbons.

9. A continuous process for producing a concentrated solution of indoles in aromatic hydrocarbons from an indole-bearing oil containing aromatic hydrocarbons which comprises extracting said indole-bearing oil with a 1,2-alkylene carbonate containing from three to four carbon atoms to produce an alkylene carbonate-indolesaromatic hydrocarbons rst extract; back-extracting said irst extract with a naphtha to remove as extract the bulk of the aromatic hydrocarbons and leave as a second resi due alkylene carbonate, indoles and aromatic hydrocarbons; extracting said second residue with an alkyl ether to produce an ether-indoles-aromatic hydrocarbons third ex- 2,916,496 7 8; tract; .distilling said ether from said third extract; Washing References Cited in the file of this patent said'third extract With Water to remove residual carbonate; UNITED STATES "PATENTS washing said third extract with aqueous caustic to remove Q 2,688,645 'Badertscher et al. 1

phenols; and washing said third extract with an aqueous mir'leraly acid to remove nitrogen bases and leave as pro- 5 Y j y V y duct said concentrated solution of indoles in aromatic 'FOREIGN PATENTS hydrocarbons, 695,479 Great Britain f'. Aug.' 12, 1953 Y 

1. A CONTINUOUS PROCESS FOR PRODUCING A CONCENTRATED SOLUTION OF INDOLES IN AROMATIC HYDROCARBONS FROM AN INDOLE-BEARING OIL CONTAINING AROMATIC HYDROCARBONS WHICH COMPRISES EXTRACTING SAID INDOLE-BEARING OIL WITH A 1,2ALKYLENE CARBONATE CONTAINING FROM THREE TO FOUR CARBON ATOMS TO PRODUCE AN ALKYLENE CARBONATE-INDOLES-ARO MATIC HYDROCARBONS FIRST EXTRACT; BACK-EXTRACTING SAID FIRST EXTRACT WITH NAPHTHA TO REMOVE AS EXTRACT THE BULK OF THE AROMATIC HYDROCARBONS AND LEAVE AS A SECOND RESIDUE ALKYLENE CARBONATE, INDOLES AND AROMATIC HYDROCARBONS; EXTRACTING SAID SECOND RESIDUE WITH A LOWER ALKY 